Method for characterizing shear wave formation anisotropy

ABSTRACT

A method of characterizing shear wave anisotropy in a formation includes obtaining crossed-dipole waveforms from a borehole penetrating the formation over a range of depths and frequencies, determining far-field slowness in a fast-shear and slow-shear direction using a low-frequency portion of the crossed-dipole waveforms, and determining near-wellbore slowness in the fast-shear and slow-shear directions using a high-frequency portion of the crossed-dipole waveforms. The method also includes marking a selected depth of the formation as having intrinsic anisotropy if at the selected depth the far-field slowness in the fast-shear direction is less than the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is less than the near-wellbore slowness in the slow-shear direction. The selected depth is marked as having stress-induced anisotropy if the far-field slowness in the fast-shear direction is less than the far-field slowness in the slow-shear direction and the near-wellbore slowness in the fast-shear direction is greater than the near-wellbore slowness in the slow-shear direction.

BACKGROUND OF THE INVENTION

The invention relates generally to formation evaluation using boreholesonic logging. More specifically, the invention relates to a method fordistinguishing between intrinsic and stress-induced anisotropy in ananisotropic formation.

A formation is said to be anisotropic if the value of a property of theformation varies with direction of measurement. A formation has shearwave anisotropy if shear wave velocity in the formation varies withazimuth. Shear wave anisotropy can be detected in a formation using acrossed-dipole sonic log obtained from a borehole penetrating theformation. The crossed-dipole sonic log is generated by measuringvelocities of two orthogonal dipole modes in the formation. Two forms ofshear wave anisotropy are considered herein: intrinsic andstress-induced. Intrinsic shear wave anisotropy may arise from intrinsicstructural effects, such as layering of shale in a deviated borehole oraligned fractures, and tectonic stresses. Stress-induced shear waveanisotropy arises from the redistribution of the far-field horizontalstresses around the borehole. Existing crossed-dipole sonic logindicates anisotropic zones of the formation but does not indicate thedominant underlying cause of the anisotropy. However, distinguishingbetween intrinsic and stress-induced anisotropy is important. Intrinsicanisotropy, specifically fracture anisotropy, plays an important role indrilling, production, and completion strategies. For example, it isdesirable that boreholes are placed in the formation such that theyintersect as many fractures as possible. Stress-induced anisotropy playsan important role in completion strategies. For example, perforationsoriented perpendicular to minimum stress direction can be used tooptimize hydraulic fracture design.

Plona et al. describe a method for distinguishing between intrinsic andstress-induced anisotropy in a formation using a crossed-dipole soniclog. (Plona T. J., et al., “Using Acoustic Anisotropy,” paper presentedat 41^(st) SPWLA Symposium: June 2000). The method exploits the factthat stress-induced dipole anisotropy in slow formations exhibitsflexural mode dispersion crossover whereas intrinsic dipole anisotropydoes not. (Plona T. J., et al., “Stress-Induced Dipole Anisotropy:Theory, Experiment and Field Data,” paper RR, presented at 40^(th) SPWLASymposium '99). The method includes obtaining crossed-dipole waveformsfrom a borehole. Alford Rotation is applied to the crossed-dipolewaveforms to identify the fast-shear direction, Flexural dispersioncurves, i.e., slowness versus frequency curves, are obtained byprocessing the rotated waveforms for dipole polarizations parallel andnormal to the fast-shear and slow-shear directions using a modifiedmatrix pencil algorithm. The slow-shear direction is orthogonal to thefast-shear direction. Slowness, measured in microseconds per foot, isthe amount of time for a wave to travel a certain distance. FIGS. 1A and1B show dispersion curves for an intrinsic anisotropic formation and astress-induced anisotropic formation, respectively. The dispersioncurves are generally parallel for an intrinsic anisotropic formation andcross for a stress-induced anisotropic formation. Although not shown inthe figures, dispersion curves coincide for an isotropic formation.

The Plona et al. method of distinguishing between intrinsic andstress-induced anisotropy requires interpretation of individualdispersion curves, which may not be efficient or practical for largedata sets. A continuous method of distinguishing between intrinsic andstress-induced anisotropy would be useful to efficiently diagnose thecause of anisotropy.

SUMMARY OF THE INVENTION

In one aspect, the invention relates to a method of characterizing shearwave anisotropy in a formation which comprises obtaining crossed-dipolewaveforms from a borehole penetrating the formation over a range ofdepths and frequencies, determining far-field slowness in a fast-shearand slow-shear direction using a low frequency portion of thecrossed-dipole waveforms, determining near-wellbore slowness in thefast-shear and slow-shear directions using a high-frequency portion ofthe crossed-dipole waveforms, marking a selected depth of the formationas having intrinsic anisotropy if at the selected depth the far-fieldslowness in the fast-shear direction is less than the far-field slownessin the slow-shear direction and the near-wellbore slowness in thefast-shear direction is less than the near-wellbore slowness in theslow-shear direction, and marking a selected depth of the formation ashaving stress-induced anisotropy if at the selected depth the far-fieldslowness in the fast-shear direction is less than the far-field slownessin the slow-shear direction and the near-wellbore slowness in thefast-shear direction is greater than the near-wellbore slowness in theslow-shear direction.

Other features and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate dispersion curves for different media.

FIGS. 2A and 2B are flowcharts illustrating a method of characterizingshear wave formation anisotropy according to one embodiment of theinvention.

FIGS. 3A and 3B illustrate a setup for acquiring crossed-dipolewaveforms from a borehole.

FIG. 4 illustrates near-wellbore and far-field regions for a borehole.

FIG. 5 shows a log obtained from Alford Rotation processing ofcrossed-dipole waveforms.

FIG. 6A shows crossed-dipole waveforms obtained at a selected depth in aborehole penetrating a slow formation.

FIG. 6B shows a contour plot of slowness vs. time for the crossed-dipolewaveforms of FIG. 6A.

FIG. 6C shows a log obtained from STC processing of crossed-dipolewaveforms.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described in detail with reference to a fewpreferred embodiments, as illustrated in accompanying drawings. In thefollowing description, numerous specific details are set forth in orderto provide a thorough understanding of the invention. However, it willbe apparent to one skilled in the art that the invention may bepracticed without some or all of these specific details. In otherinstances, well-known features and/or process steps have not beendescribed in detail in order to not unnecessarily obscure the invention.The features and advantages of the invention may be better understoodwith reference to the drawings and discussions that follow.

FIG. 2A is a flowchart illustrating a method of characterizing shearwave formation anisotropy according to one embodiment of the invention.The method includes acquiring crossed-dipole waveforms from a boreholepenetrating a formation as a function of frequency and depth in theborehole (200). The method further includes determining the fast-sheardirection or azimuth (202). Methods for determining the fast-sheardirection include, but are not limited to, Alford Rotation andparametric inversion of the crossed-dipole waveforms. The slow-sheardirection is orthogonal to the fast-shear direction. The method furtherincludes determining far-field slowness in the fast-shear and slow-sheardirections (204). The method further includes determining near-wellboreslowness in the fast-shear and slow-shear directions (206). For aselected interval of the formation, the method includes distinguishingbetween intrinsic and stress-induced anisotropy by comparing thefar-field and near-wellbore slownesses in the fast-shear and slow-sheardirections (208). If the interval of the formation has intrinsicanisotropy, the shear slownesses in the fast-shear and slow-sheardirections will be consistent from the near-wellbore to the far-field(i.e., parallel dispersion curves). If the interval of the formation hasstress-induced anisotropy, the shear slownesses in the fast-shear andslow-shear directions will not be consistent from the near-wellbore tothe far-field (i.e., crossing dispersion curves). The method of theinvention avoids advanced dispersion analysis by simply comparing thefar-field and near-wellbore slownesses for the fast-shear and slow-sheardirections in the time domain.

FIG. 3A illustrates a setup for acquiring crossed-dipole waveforms froma borehole 300 penetrating a subterranean formation 302. Thecrossed-dipole waveforms are acquired as a function of frequency anddepth in the borehole 300. It should be noted that only the parts of thesetup relevant to the understanding of the invention are shown anddescribed. The borehole 300 may be a vertical hole or a deviated holeand is filled with fluid or drilling mud. A logging tool 304 is disposedin the borehole 300. For measurement purposes, the logging tool 304 maybe conveyed to a desired depth in the borehole 300 in a number of ways,including, but not limited to, on the end of a wireline, coiled tubing,or drill pipe. For illustration purposes, the logging tool 304 is shownat the end of a wireline 306. The length of the wireline 306 may providean estimate of the depth of the logging tool 304 in the borehole 300.The wireline 306 may also be used to provide communication between thelogging tool 304 and a surface system 307. The surface system 307 mayinclude a processor which executes an algorithm for characterizing shearwave formation anisotropy, as outlined in FIGS. 2A and 2B.

The logging tool 304 can be any tool that can provide borehole shearslowness along two orthogonal directions, such as one available underthe trade name Dipole Shear Imager (DSI) tool from Schlumberger. Forillustration purposes, the logging tool 304 includes dipole sources 308,310. The dipole sources 308, 310 are in orthogonal relation to eachother and may or may not be on the same plane. The logging tool 304 mayinclude an isolation joint 312 to prevent signals from the dipolesources 308, 310 from traveling up the tool. The dipole sources 308, 310may be any source suitable for shear/flexural logging, such aspiezoelectric ceramics made in benders or cylindrical sections,magnetostrictive transducers, and electrodynamic vibrators. In oneembodiment, the dipole source 308 generates flexural waves at arelatively low frequency, and the dipole source 310 generates flexuralwaves at a relatively high frequency. The low and high frequencies arepreferably chosen such that if a dispersion crossover occurs it would bedetectable. However, this does not mean that a dispersion analysis isrequired for practice of the invention. On the other hand, existingdispersion curves can provide general information on radial gradient ofshear slowness, which can assist in selecting operating frequencies ofthe dipole sources 308, 310. In general, long wavelength/low frequencyprobes deep and short wavelength/high frequency probes shallow.

Preferably, the relatively low frequency of the dipole source 308 ischosen such that the far-field region of the borehole 300 is probed.Preferably, the relatively high frequency of the dipole source 310 ischosen such that the near-wellbore region of the borehole 300 is probed.The depth of investigation is proportional to the wavelength, which is afunction of velocity and frequency, i.e., λ=v/f, where λ is wavelength;V is velocity, and f is frequency. Velocity and frequency depend on theformation characteristics and borehole diameter. FIG. 4 illustrates anear-wellbore region 400 and a far-field region 402 for a borehole 404.Generally, the near-wellbore region 400 is about ½ borehole diameter,measured radially from the surface 404 a of the borehole 404. If theborehole diameter is 12 in., for example, then the near-wellbore region400 would be about 6 in. measured radially from the surface 404 a of theborehole 404. For many formations, approximately 4-7 kHz would probe thenear-wellbore region. Generally, the far-field region 402 is about 2-3borehole diameters, measured radially from the surface 404 a of theborehole 404. If the borehole diameter is 12 in., for example, then thefar-field region 402 would be about 24 in. to 36 in. measured radiallyfrom the surface 404 a of the borehole 404. For many formations,approximately 1-3 kHz would probe the far-field region. However, theinvention is not limited to these frequency ranges. For example,approximately 4-12 kHz could be used to probe the near-wellbore region,and approximately 1-3.5 kHz could be used to probe the far-field region.

Returning to FIG. 3A, the logging tool 304 includes a plurality ofspaced-apart receiver stations 314. As shown in FIG. 3B, each receiverstation 314 includes four dipole receivers 314 a, 314 b, 314 c, and 314d. The dipole receivers 314 a, 314 c form a pair and are oriented inlinewith the dipole source 308 and orthogonal to the dipole source 310, andthe dipole receivers 314 b, 314 d form a pair and are oriented inlinewith the dipole source 310 and orthogonal to the dipole source 308. Thisarrangement allows detection of flexural wave signals in the fast-shearand slow-shear directions. The dipole receivers 314 a, 314 b, 314 c, and314 d may be any type of dipole transducer that detects pressuregradients or particle vibrations, such as hydrophones, benders, andelectrodynamic transducers, and is sensitive in the frequency range ofthe dipole sources (308, 310 in FIG. 3A). Although this figure showsjust four receivers, the receiver station could consist of any number ofreceivers, for example eight receivers arranged azimuthally with 45degree separation, thus including the detection of flexural wave signalsfrom modal decomposition.

Returning to FIG. 3A, the logging tool 304 also includes an electronicscartridge 316 which includes circuitry to power the dipole sources 308,310 and receiver stations 314 and to process signals received at thereceiver stations 314. Such processing may include digitizing theseparate waveforms received at the receiver stations 314 and stackingthe waveforms from multiple firings of the dipole sources 308, 310. Theelectronics cartridge 316 may further transmit the processed signals tothe surface system 307 or store the processed signals in a downholememory tool (not shown), in which case the data can be retrieved whenthe logging tool 304 is pulled to the surface.

In operation, the dipole sources 308, 310 emit dipole acoustic signalswhich excite flexural wave frequencies in the formation 302. The dipolereceivers 314 detect dipole acoustic signals from the formation 302. Thelogging tool 304 rotates in the borehole 300 so that the dipole sources308, 310 can be fired at different azimuthal positions around theborehole 300. The crossed-dipole waveforms recorded by the dipolereceivers 314 generally have a multitude of arrivals, often including acompressional arrival, a shear arrival, and a flexural mode arrival. Theflexural mode arrival dominates the borehole response and is dispersiveand is most suitable for processing. However, other modes could beprocessed as well. Excitation of the borehole 300 at an arbitraryazimuthal orientation results in two shear waves if anisotropy ispresent, one propagating as a fast-shear wave and another propagating asa slow-shear wave.

Each crossed-dipole waveform received at one of the receiver stations314 has four components produced from inline and orthogonal orientationof each receiver pair (314 a, 314 c and 314 b, 314 d in FIG. 3B) witheach of the dipole sources 308, 310. The method according to oneembodiment of the invention includes determining the fast-sheardirection or azimuth from these four-component crossed-dipole waveforms(202 in FIG. 2A). Methods for determining the fast-shear directioninclude, but are not limited to, Alford Rotation and parametricinversion of the crossed-dipole waveforms. The slow-shear direction issimply orthogonal to the fast-shear direction.

Alford rotation is described in, for example, Alford, R. M., 1986, Sheardata in the presence of azimuthal anisotropy. 56^(th) AnnualInternational Meeting, Society of Exploration Geophysicists, ExpandedAbstracts, 476-479, and U.S. Pat. Nos. 4,803,666, 4,817,061, 5,025,332,4,903,244, and 5,029,146, the contents of which are incorporated hereinby reference. Generally speaking, Alford rotation involves choosing anumber of rotation angles, applying these rotation angles to thefour-component crossed-dipole waveform data, and finding an angle thatminimizes the energy in the mismatched components (orcross-line/off-line components).

FIG. 5 shows an example of a log produced from Alford Rotationprocessing of crossed-dipole waveforms. The raw waveforms are shown at500. The difference between minimum and maximum cross-line energyresulting from the mismatched components, which is the end result of theAlford Rotation processing for determining the fast-shear direction, isshown at 502. The fast-shear direction, which is determined based on theminimization of the cross-line components, is shown at 504. Track 506represents the raw waveforms 500 rotated into the fast-shear andslow-shear directions. The slow-shear direction is orthogonal to thefast-shear direction. Track 508 shows the difference between fast-shearand slow-shear slowness of rotated waveforms. Track 510 shows thedifference between fast and slow arrival times of rotated waveforms. Itshould be noted that the slownesses are presented only at lowfrequencies (1-3 kHz), but the invention involves Alford Rotation oflow- and high-frequency portions of the crossed-dipole waveforms.

The method according to one embodiment of the invention includesdetermining far-field slowness in the fast-shear and slow-sheardirections (204 in FIG. 2A). Far-field slowness in the fast-shear andslow-shear directions can be determined from the low-frequency portionof the rotated crossed-dipole waveforms using, for example,Slowness-Time-Coherence (STC) analysis, also known as semblanceprocessing. STC involves identifying and measuring the slowness and timearrival of coherent energy propagating across an array of receivers. Thetechnique includes passing a narrow window across the waveforms receivedat the receiver stations and measuring the coherence within the windowfor a wide range of slowness and times of arrivals. STC is described in,for example, Kimball, C. V., Shear slowness measurement by dispersiveprocessing of the borehole flexural mode: Geophysics, Vol. 63, No. 2, p.337-344. The same process can be used to determine near-wellboreslowness in the fast-shear and slow-shear directions (206 in FIG. 2A),except in this case STC is applied to the high-frequency portion of therotated crossed-dipole waveforms. FIG. 6A depicts crossed-dipolewaveforms at a depth X50 in a borehole penetrating a slow formation,taken with an eight-receiver array, with 0.5 ft (0.152 m) spacingbetween the receivers. FIG. 6B shows a contour plot of slowness versustime for the crossed-dipole waveforms shown in FIG. 6A. The slownessversus time is obtained from STC processing of the crossed-dipolewaveforms. FIG. 6C shows a log produced by STC processing ofcrossed-dipole waveform data for the borehole of FIG. 6A for depths X30to X90. The track 600 represents slowness as a function of depth.

Once the far-field and near-wellbore slownesses are determined, theprocess for distinguishing between intrinsic and stress-inducedanisotropy is quite simple. As previously mentioned, this involvescomparing the far-field and near-wellbore slownesses in the fast-shearand slow-shear directions (208 in FIG. 2A). The test is illustrated inFIG. 2B. A depth of the formation is selected (208 a). For the selecteddepth, if the fast-shear slowness in the far-field (low frequency) isless than the slow-shear slowness in the far-field (208 b) and if thefast-shear slowness in the near-wellbore (high frequency) is less thanthe slow-shear slowness in the near-wellbore (208 c), then the formationat the selected depth is marked as having intrinsic anisotropy. For theselected depth, if the fast-shear slowness in the far-field (lowfrequency) is less than the slow-shear slowness in the far-field (208 b)and if the fast-shear slowness in the near-wellbore (high frequency) isgreater than the slow-shear slowness in the near-wellbore (208 d), thenthe formation at the selected depth is marked as having stress-inducedanisotropy. It follows from the above that the selected interval of theformation is isotropic if the fast-shear slowness and slow-shearslowness in the far-field are the same and if the fast-shear slownessand slow-shear slowness in the near-wellbore are the same. The methodmay also include marking a selected depth of the formation as havingisotropic anisotropy.

The invention typically provides the following advantages. The methodallows continuous processing of crossed-dipole waveform data tocharacterize shear wave formation anisotropy. Shear wave formationanisotropy can be characterized without advanced dispersion analysis.The fast-shear and slow-shear slownesses in a stressed-inducedanisotropic zone are proportional to the minimum and maximum horizontalstress, which allows for quantification of these stresses. This allowsfor three-dimensional stress inversion modeling for reservoirstimulation, drilling optimization, and hydraulic fracture stimulation.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

1.-17. (canceled)
 18. A system configured to characterize shear waveanisotropy in a formation, comprising: a logging tool configured to:obtain crossed-dipole waveforms from a borehole penetrating theformation over a range of depths and frequencies; and a surface unitoperatively connected to the logging tool and configured to: determinefar-field slowness in a fast-shear direction and slow-shear directionusing a low-frequency portion of the crossed-dipole waveforms; determinenear-wellbore slowness in the fast-shear direction and slow-sheardirection using a high-frequency portion of the crossed-dipolewaveforms; select a depth in the formation; characterize the depth ofthe formation as having intrinsic anisotropy when at the depth thefar-field slowness in the fast-shear direction is less than thefar-field slowness in the slow-shear direction and the near-wellboreslowness in the fast-shear direction is less than the near-wellboreslowness in the slow-shear direction; and characterize the depth of theformation as having stress-induced anisotropy when at the depth thefar-field slowness in the fast-shear direction is less than thefar-field slowness in the slow-shear direction and the near-wellboreslowness in the fast-shear direction is greater than the near-wellboreslowness in the slow-shear direction.
 19. The system of claim 18,wherein the surface unit is further configured to: determine thefast-shear direction prior to determining the far-field slowness and thenear-wellbore slowness, wherein the slow-shear direction is orthogonalto the fast-shear direction.
 20. The system of claim 19, whereindetermining the fast-shear direction comprises Alford Rotationprocessing of the crossed-dipole waveforms.
 21. The system of claim 19,wherein determining the fast-shear direction comprises parametricinversion of the crossed-dipole waveforms.
 22. The system of claim 18,wherein obtaining crossed-dipole waveforms comprises firing a pluralityof dipole sources located on the logging tool to generate dipoleacoustic signals which are transmitted into the formation.
 23. Thesystem of claim 22, wherein obtaining crossed-dipole waveforms furthercomprises firing the plurality of dipole sources at different azimuthalpositions in the borehole.
 24. The system of claim 22, wherein obtainingcrossed-dipole waveforms further comprises detecting dipole acousticsignals from the formation using a plurality of dipole receivers locatedon the logging tool.
 25. The system of claim 24, wherein a first set ofthe dipole receivers selected from the plurality of dipole receivers areinline with a first one of the plurality of dipole sources and a secondset of the dipole receivers selected from the plurality of dipolereceivers are inline with a second one of the plurality of dipolesources.
 26. The system of claim 22, wherein a first one of theplurality of dipole sources fires at a low frequency and a second one ofthe plurality of dipole sources fires at a high frequency.
 27. Thesystem of claim 26, wherein the low frequency is in a range fromapproximately 1 to 3 kHz.
 28. The system of claim 26, wherein the highfrequency is in a range from approximately 4 to 7 kHz.
 29. The system ofclaim 26, wherein the low frequency and the high frequency are selectedsuch that dispersion crossover would be detectable if dispersion curveswere generated from the crossed-dipole waveforms.
 30. The system ofclaim 26, wherein the high frequency is selected to probe into theformation a radial distance of approximately one-half the boreholediameter.
 31. The system of claim 26, wherein the low frequency isselected to probe into the formation a radial distance of approximatelytwo to three times the borehole diameter.
 32. The system of claim 18,wherein determining far-field slowness involves processing thecrossed-dipole waveforms using slowness-time-coherence.
 33. The systemof claim 18, wherein determining near-wellbore slowness involvesprocessing the crossed-dipole waveforms using slowness-time coherence.34. A system configured to characterize shear wave anisotropy in aformation, comprising: a logging tool configured to: obtaincrossed-dipole waveforms from a borehole penetrating the formation overa range of depths and frequencies; and a surface unit operativelyconnected to the logging tool and configured to: determine far-fieldslowness in a fast-shear direction and slow-shear direction using alow-frequency portion of the crossed-dipole waveforms; determinenear-wellbore slowness in the fast-shear direction and slow-sheardirection using a high-frequency portion of the crossed-dipolewaveforms; select a depth in the formation; and characterize the depthas having isotropic anisotropy when at the depth the far-field slownessin the fast-shear direction is substantially the same as the far-fieldslowness in the slow-shear direction.
 35. The system of claim 34,wherein the surface unit is further configured to: characterize thedepth as having isotropic anisotropy when at the depth the near-wellboreslowness in the fast-shear direction is substantially the same as thenear-wellbore slowness in the slow-shear direction.
 36. A systemconfigured to characterize shear wave anisotropy in a formation,comprising: a logging tool configured to: obtain crossed-dipolewaveforms from a borehole penetrating the formation over a range ofdepths and frequencies; and a surface unit operatively connected to thelogging tool and configured to: determine far-field slowness in afast-shear direction and slow-shear direction using a low-frequencyportion of the crossed-dipole waveforms; determine near-wellboreslowness in the fast-shear direction and slow-shear direction using ahigh-frequency portion of the crossed-dipole waveforms; select a depthin the formation; and characterize the depth as having isotropicanisotropy when at the depth the far-field slowness in the fast-sheardirection is substantially the same as the far-field slowness in theslow-shear direction and the near-wellbore slowness in the fast-sheardirection is substantially the same as the near-wellbore slowness in theslow-shear direction.